1. Field of the Invention
This invention relates to a process for the production of semiconductor devices, in which a SOI (Silicon on Insulator) technique is employed with a laser beam.
2. Description of the Related Art
To produce high speed integrated circuits and/or three-dimensional integrated circuits, research and development for SOI techniques, by which a single crystalline silicon layer can be formed on a silicon substrate covered with an insulating layer, have been carried out. One of the SOI techniques is a laser recrystallization method, which is especially applicable to the production of three-dimensional integrated circuits. The laser recrystallization method comprises forming a silicon oxide film on a silicon substrate and forming an amorphous or polycrystalline silicon film on the silicon oxide film, followed by irradiating the amorphous or polycrystalline silicon film with a scanning laser beam to achieve a single crystallization of the amorphous or polycrystalline silicon film. According to this method, in order to melt the silicon film by the irradiation of the scanning laser beam, the laser beam should be focused into a spot with a diameter of approximately 100 .mu.m and scanned on the whole surface of the wafer. In order that a single crystalline region is created during recrystallization of the silicon film, the trailing edge (i.e., the interface between the solid region and the molten region) of the molten region of the silicon film must be concaved toward the molten region. An approach for attaining such a concaved trailing edge is essential for producing a single crystalline region.
There have been two kinds of approaches for the formation of the concave trailing edge of the molten region. For one of the two approaches, the silicon film to be recrystallized is made of a particular lamination structure to control the reflectivity and/or the thermal conductivity to the substrate, resulting in a concaved trailing edge. This approach is disadvantageous in that the single crystalline region must have a width of as small as approximately 20 .mu.m since the trailing edge must be changed from convex to concave due to the particular structure of the sample film. This causes power losses.
In another approach, the Gaussian distribution of the laser beam is transformed into a dual peak type power distribution with peaks at both ends and a valley in the center portion, and then the silicon film is irradiated with the laser beam having a dual type power distribution resulting in a concaved trailing edge. When the Gaussian beam (FIG. 7(a)) is used for the irradiation of the sample film, the film is molten and a number of grains start to grow from both ends of the molten region of a low temperature to the center portion thereof of a high temperature, resulting in aggregation of polycrystals as shown in FIG. 14. Thus, the laser beam used for the achievement of a single recrystallization must exhibit a dual peak type power distribution rather than the Gaussian distribution. However, this approach is disadvantageous in that power losses of the laser beam are sometimes great, and even when the power losses are quite small, this approach is too sensitive to variations in the power distribution of the laser beam to stably create a single crystalline region so that subboundaries and/or twins tend to occur therein.